A general CMOS (Complementary Metal Oxide Semiconductor) solid-state imaging device has a mechanism in which a pixel array arranged two-dimensionally is scanned sequentially for each pixel row to read out pixel signals. This row-sequential scanning causes time displacement in an accumulation period for each pixel row, thereby causing a phenomenon called focal plane distortion that the image obtained by photographing a moving subject is distorted.
In the photographing of a subject moving fast, for which such image distortion is not allowed, or in the application of sensing in which the simultaneity of the photographed image is necessary, the global shutter function or the mechanical shutter function has been suggested for achieving the simultaneity of the accumulation period of the pixel array.
The mechanical shutter function enables the global exposure in which the exposure is finished simultaneously in the entire pixels by controlling the exposure time with a mechanical light-shielding means. Specifically, a mechanical shutter is opened to start the exposure simultaneously for the entire pixels, and is closed after a certain period of times to end the exposure. In the case of performing the global exposure with the mechanical shutter, however, a mechanical light-shielding means is necessary, which makes it difficult to reduce the size. Further, since there is a limitation in mechanical driving speed of the mechanical shutter, the simultaneity of the exposure time in the pixel region is poor.
On the other hand, the global shutter function enables the global exposure in which the exposure is finished simultaneously in the entire pixels by electrical control. Specifically, the accumulation of the signal charges in the entire pixel array surface is started simultaneously by performing the entire-row simultaneous reset driving of photodiodes in the pixel array. Then, by the entire-row simultaneous transfer driving to the charge accumulation unit such as floating diffusion, the accumulation of signal charges in the entire pixel array surface is finished simultaneously.
Incidentally, in the case of employing the global shutter function, the signal charges are read out by the row sequential scanning in a manner similar to the general CMOS solid-state imaging device. Therefore, it is necessary to accumulate the signal charges, which have been read out by the global shutter function simultaneously in the entire pixels, in the charge accumulation unit such as the floating diffusion until the readout time. This causes problems that the signal charges held in the floating diffusion until the readout time leak out or deteriorate due to noise (referred to as alias) that is generated by the photoelectric conversion of the floating diffusion.
For preventing this, a structure has been suggested in which a light-shielding film is provided over the charge accumulation unit such as the floating diffusion. The provision of the light-shielding film, however, causes problems that the sensitivity deteriorates because the opening area of the photodiode is reduced and moreover that the saturated sensitivity deteriorates. Moreover, since the charge accumulation unit such as the floating diffusion is often disposed at the position close to the photodiode, which serves as a light incidence portion, in the horizontal direction, the light may leak therein due to the light diffraction phenomenon or the scattering phenomenon, in which case the noise increases.
Furthermore, at an interface between silicon and a silicon oxide film, a large number of crystal defects are present, so that dark current easily occurs. Therefore, when the charges are held in the floating diffusion, a difference is generated in the dark current applied to the signal level depending on the order of readout. The noise from such dark current cannot be cancelled just by the noise removal by the reset level.
As a solution to this problem, a solid-state imaging device has been suggested in which a memory unit that accumulates charges is mounted separately from the floating diffusion in a pixel (Patent Documents 1 and 2). The memory unit is formed as a charge accumulation unit that can be embedded, and temporarily holds the signal charges transferred from the photodiodes. In the structure having the memory unit as above, however, the area where the light is blocked is large relative to the pixel area, so that the opening area becomes smaller. As a result, the sensitivity is deteriorated further.
On the other hand, a solid-state imaging device of back-illumination type has been suggested as a means for increasing the opening area of the pixel relative to the incident light. In the solid-state imaging device of back-illumination type, the opening of the pixels can be increased by having, as the light incidence surface, the back surface side that is on the opposite side of the front surface of a semiconductor substrate provided with circuits including transistors, wires, etc., and the miniaturization is thus enabled.
Incidentally, in the case of adding the global shutter function in the solid-state imaging device of back-illumination type, a structure is employed in which a light-shielding film is formed on the light incident surface side for preventing the light from entering to reach the front surface side of the semiconductor substrate. In this case, when the light-shielding film is formed largely, the opening area of the pixels is narrowed to make the miniaturization difficult.
Patent Document 3 has disclosed the structure in which a capacitor is formed outside a semiconductor substrate. In this structure, however, a large amount of dark current is generated from the capacitor and the high-quality image cannot be obtained.